This invention relates to improvements in hollow semipermeable fiber elements of the type disclosed in Mahon U.S. Pat. No.3,228,876. Such elements are adapted for use in industrial osmosis, ultrafiltration or dialysis processes, and are particularly useful in medical applications including blood oxygenation, and purification by hemodialysis or hemofiltration.
Mahon type semipermeable fiber elements have achieved their greatest commercial utilization in artificial kidney devices having the general configuration of a tube and shell heat exchanger similar to that shown in U.S. Pat. No. 3,228,377. Such devices couple, or seal, the hollow fiber bundle into a tubular shell by using a solidified castable resin tubesheet on each end of the fiber bundle and sealing the outer rim of the tubesheet to the inner wall of the shell. The outer end of the tubesheet is transversely severed to form an outer end planar surface which exposes the open fiber ends, and that planar surface then becomes the inner wall of an outwardly extending contiguous blood chamber.
Tubesheets for artificial kidneys are typically formed by centrifugally casting a castable synthetic resin around the fiber end portions while the fiber bundle is positioned within the shell such that the casting resin solidifies within the end portions of the shell and concurrently adheres to and seals the rim portion of the solid tubesheet to the inner walls of the shell. This procedure has been successful for combinations of certain resin shells and certain castable resins adhesive to those shells, notably thermosetting resins of the epoxy and polyurethane type as disclosed in U.S. Pat. Nos. 3,619,459; 3,703,962; and 3,962,094. Prior to this invention, however, to our knowledge, no one has successfully employed thermoplastic resins in a process of centrifugally casting tubesheets in the manufacture of artificial kidneys. The only known use of thermplastic resins in artificial kidney tubesheet manufacture is that disclosed in Tigner U.S. Pat. No. 4,138,460.
It has long been recognized that thermoplastic resins offer advantages as tubesheets for hollow fibers that are not possessed by thermosetting resins. Thermoplastic resins offer faster potting cycle times than thermosetting resins and thereby reduce the overall production time required to manufacture an artificial kidney ready for testing; thermoplastic resins are free from noxious vapor or gas generation during casting which may occur with certain epoxy and polyurethane resins; thermoplastic resins are cheaper than thermosets and thermoplastic tubesheet-shell devices can be ethylene oxide sterilized easier and in less time than corresponding thermoset tubesheet-shell devices.
On the other hand thermoplastic resins have entirely different handling characteristics than thermosetting tubesheet resins and these characteristics have, prior to this invention, prevented successful adoption of thermoplastics for centrifugal casting of hollow fiber tubesheets. The handling characteristics referred to stem from the basic reaction of thermoplastics to temperature changes and this reaction creates formidable problems when combined with the necessity to penetrate between and to wet the external wall surfaces of thousands of capillary size hollow fibers and thereafter to solidify into a sound, internal-void-free tubesheet. Epoxy and polyurethane resins or polymers, become polymers by chemical reaction between initially fluid multi-components, or comonomers, and generate heat during reaction; the generated heat causes the reaction product to solidify, or set, at or above some high threshold temperature. In contrast, thermoplastic resins are polymers that are solid at room temperature but soften and become liquid as the temperature rises past a threshold value; molten thermoplastic resins then solidify as the temperature is reduced and passes through the threshold temperature on the way back down toward room temperature. Thermoplastics also have a substantially greater shrinkage during solidification than thermosetting resins. Thus, translating these thermoplastic resin characteristics into required handling conditions during centrifugal casting, it is necessary to, first, raise the temperature of the selected thermoplastic resin to convert it into a liquid, preferably a low viscosity liquid, and during casting to control the temperature of the entire molten mass of resin so as to insure penetration into and around each fiber in the bundle; the condition that must be avoided is localized temperature drop below the solidification threshold temperature and a resultant flow blockage anywhere in the inflow path of the resin prior to the arrival of liquid resin at the furthermost contemplated point from the infeed location. Second, the shrinkage of the mass of the cast liquid resin confined within a mold during solidification must be controlled so as to counteract the resin tendency to contract from the liquid pool toward each solidifying location in the mass. It is also necessary to recognize that the volume of thermoplastic resin shrinkage is so great that it has not been found to be possible to solidify a disc-shaped tubesheet within a tubular shell and retain a sound, non-fractured, adhesion seal between the rim of the tubesheet and the inner wall of the shell as is routinely achieved with the commercially used thermosetting tubesheet resin compositions. This failure of the thermoplastic tubesheet to adhere to the tubular shell wall is serious because it necessitates formation of a separate seal between the shell and tubesheet in some other fashion to separate the shell into the desired three separate fluid-tight isolated zones, e.g., in the case of an artificial kidney, a central dialysate zone between two spaced apart end blood chambers.
In artificial kidneys employing thermosetting resin tubesheets it is conventional to form the blood chambers by sealing a generally cup-shaped member against the planar outer end surface of the tubesheet by a conventional circular O-ring as illustrated in FIG. 4 of U.S. Pat. No. 3,882,024. With thermoplastic tubesheets, such blood chamber constructions are not feasible due to inability to form an effective seal between the thermoplastic tubesheet and the inner shell wall.
Prior to this invention all of the problems above identified have remained unsolved.
It is therefore the principal object of this invention to provide an iimproved hollow fiber semipermeable membrane separatory element having an integral tubesheet on each end of a solidified castable resin; each tubesheet has a solid, axially extending disc section terminating in an outer end planar surface exposing the open ends of the fibers therein, and a radially outwardly tapering surface extending from the inner end surface toward the outer portion thereof; the tubesheet optionally includes a second tapered portion that extends outwardly from the outer face of the disc portion and its peripheral surface tapers radially inwardly toward the outer end planar surface which exposes the open ends of the fibers therein.
A second important object is to provide a separatory device which incorporates the new separatory element of this invention and provides improved means for sealing that element into a surrounding shell, or jacket.
Another object of this invention is to provide a method suitable for centrifugally casting thermoplastic castable resin tubesheets on each end of a bundle of hollow semipermeable fibers to form the new separatory element of this invention.